This is a snap shackle (aka quick-release shackle)!
Its purpose is to be able to easily open the shackle even though it is currently under a load. Its designed to be tough, but It's still just plastic: so, it will only survive light to medium loads. Naturally, it’s better to have a metal one, but if you don't have one, then you don't have one :P
In my particular usage, I use a similarly (professionally) designed stainless steel shackle to stow our spinnaker halyard while during the upwind section of a race. However, the boat I'm on right now didn't have one... so I made one :)
I used triangular 50% infill with 3 shells. This makes it quite tough without causing the jaws to be too rigid. The greater the infill the stronger the latch and hinge will be and the heavier the load it can sustain, but it will be harder to pop open.
You CAN print it as low as 10% infill, but it will be weak enough to pull apart yourself with a bit of umph.
Side note, since this is a load bearing and semi-flexible design, ABS is the ideal material.
The hinge will usually be stuck in place after printing. Just lightly force the jaws open and it should gently crack free - no tools required.